Perforated T-Steel Profile
It would be counter productive to use boolean in your case, you'd better create your shape and array it along a curve, like that:

Create a plane and ceate some edge loops:



Cut a hole, select the edge loop, extrude inwards, make a circle with AltShiftS:



Extrude up:




Give your object an Array and a Curve modifier. To get nice round holes you'll need to give a Subdivision Surface modifier, then you'll need to add some edge loops to sharp the edges: